Inter-apparatus communications conforming to the communication standard 100 BASE-TX or 1000 BASE-T have been widely used. Known methods for controlling an apparatus that performs inter-apparatus communications include disposition of input/output terminals for control and transmission of control signals over main lines.
However, the disposition of input/output terminals for control increases the cost, as well as requires space for disposing the input/output terminals. Further, the transmission of control signals over main lines reduces the capacity of the main lines.
Technologies for solving these problems include a transmission system disclosed in Patent Literature 1. In the transmission system of Patent Literature 1, common-mode signals are transmitted over a Ethernet® pair wire. Thus, control signals are transmitted without having to dispose input/output terminals for control and without reducing the capacity of the main lines. Since this configuration allows common-mode signals to be transmitted over each wire of the twisted pair, the communication capacity can be increased.